钼基体高温抗氧化Mo-W-Si-N复合涂层中的钨合金化作用

Molybdenum and molybdenum alloys, because of poor oxidation resistance at low and elevated temperature in oxidizing environments, have been severely limited in the application as high temperature structure materials. Study on MoSi2 composite coatings has become the key to improve the high temperature oxidation resistance of Mo and its alloys. Meanwhile, MoSi2 composite coatings are also paid extensive attention to the protection of refractory metals (such as W and Nb) and their alloys, Nickel-based alloys and C/C composites etc in oxidative atmospheres at high temperature. Combining with the valence electron structures of WSi2 and MoSi2, the diffusion of Si, high-temperature thermal stability of (Mo, W)Si2 and W-Mo infinite solid solution, an idea to design a new coating of Mo-Si-N system alloying with tungsten on Mo substrate is put out in this project. The synthesis technology of Mo-W-Si-N composite coatings by pack cementation, and alloying effect of W on oxidation performance of the coatings in air at 400-700 C and 1300-1600 C are deeply investigated. By analyzing the oxidation dynamic behaviors at low or high temperature oxidation and the evolution of phase and microstructure of the coating materials, alloying effect of W is clearly illustrated. The above investigation will enrich the design theory of MoSi2 composite coatings for high temperature antioxidation. It also give a theoretical basis to improve the operating temperature and life of refractory metals and their alloys, high temperature alloys, C/C composite materials and so on in oxidizing environments, which will expand their field of application.

钼及钼合金高温易氧化的特性极大限制了其作为高温材料的应用。MoSi2复合涂层体系的研究已成为提高钼及钼合金高温抗氧化性能的重点。同时，MoSi2也作为W和Nb等难熔金属及其合金、Ni基高温合金和C/C复合材料等的高温氧化防护涂层材料，受到广泛关注。本课题结合WSi2和MoSi2的价电子结构、Si的扩散规律、(Mo,W)Si2的高温热稳定性和W-Mo无限固溶等特点，率先提出W合金化Mo-Si-N系复合涂层的设计思想；系统考察钼基体表面制备该涂层的合理的包埋法工艺、W合金化对涂层材料在空气中400-700℃低温氧化性能和1300-1600℃高温氧化性能的影响，分析该涂层材料的高低温氧化动力学行为及相和组织的演变规律，阐明W的合金化作用。它将丰富高温抗氧化MoSi2复合涂层的设计理论，为提高难熔金属及其合金、高温合金、C/C复合材料等在氧化环境中的使用温度和服役寿命及扩大其应用范围提供理论依据。

针对钼及其合金的高温易氧化特性极大限制其作为高温材料应用的难题。本项目提出了制备钼及其合金表面Mo-W-Si-N系复合涂层的设计思想；系统考察了其包埋法工艺、W合金化涂层材料在400-700℃和1300-1600℃氧化性能，分析了高、低温氧化动力学行为及相和组织的演变规律，阐明了W的合金化作用机理。得出如下重要结果：.1、提出了钼表面Mo-W-Si-N系涂层的三步法制备工艺：渗钨合金化1400℃保温10 h+渗N1000℃保温3 h+渗Si1200℃保温20 h。.2、评价了Mo-W-Si-N系涂层的氧化性能，揭示了其氧化失效机理。.（1）涂层具有优异的高/低温抗氧化性能：在500 ℃和600 ℃的氧化速率分别为0.0145 g/(m2•h)和0.0191 g/(m2•h)，归因于表面形成连续致密的SiO2氧化膜；1600℃下的抗氧化寿命长达288h，分别是MoSi2-Si3N4和MoSi2-CrSi2-Si3N4涂层寿命的5倍和3.5倍。.（2）涂层氧化失效机理表现为氧化时，由于Si的扩散作用及其与基体的反应导致(Mo,W)5Si3和(Mo,W)3Si中间扩散层的形成；(Mo,W)Si2层全部退化时，Mo-W-N-Si系涂层失效。.（3）Mo5Si3中间层存在两种形成机制，即由于Si的扩散导致MoSi2相退化为Mo5Si3和游离Si与Mo反应生成Mo5Si3。.3、阐明了W合金化的作用机理.（1）W合金化导致涂层中生成WSi2相，强烈阻碍了氧化过程中Si的扩散。WSi2最佳含量为5 vol %， Si扩散激活能提高了约38 kJ/mol。.（2）W合金化导致Mo-W-Si-N系的CTE较接近于钼基体的CTE，其抗热震性能最好，避免了氧化过程中裂纹的产生。涂层中WSi2和Si3N4的最佳含量分别为6vol.%和30vol.%。.4、应用本项目的涂层制备技术成功在铌合金表面制备了致密均匀、结合良好、高温氧化性能较好的NbSi2涂层。